A variety of techniques exists for sharing files, printers, and other resources between two computers on a network. For example, two application-layer network protocols for sharing resources are Server Message Block (SMB) and Network File System (NFS). SMB is used by MICROSOFT™ WINDOWS™ and other operating systems to allow two computers or other resources to communicate, request access to resources, specify intended access of resources (e.g., reading, writing, etc.), lock resources, and so on. MICROSOFT™ WINDOWS™ Vista introduced SMB 2.0, which simplified the command set of SMB 1.0 and added many other enhancements. MICROSOFT™ WINDOWS™ 7 and Server 2008 R2 introduced SMB 2.1, which added opportunistic locking (oplocks) and other enhancements.
Most protocols for remote sharing of resources assume a one-to-one relationship between connections and sessions. A session represents the lifetime of any single request to access a resource and the subsequent access of that resource until the connection is terminated. A session may also be associated with a particular security principal and validated security credentials that determine the actions that are authorized during the session. A connection can include a Transmission Control Protocol (TCP). User Datagram Protocol (UDP), or other type of connection over which higher-level protocols like SMB and NFS can communicate to carry out commands. An SMB or NFS session typically involves opening a TCP or UDP connection between a source of a request and a target of the request, sending one or more SMB or NFS commands to access the target resource, and then closing the session. Sometimes connections are lost during a session (e.g., due to a network failure), tearing down any client and server state established during the connection. To reestablish a connection the client and server typically have to repeat all of the steps used to initially establish the connection over again.
The SMB2 protocol provides a resume key that allows clients to quickly reestablish a file handle to a server if a client is disconnected from the server, enabling clients to reduce network round trips to the server and reduce the load on the server when a client reconnects. However, today the resume key does not provide restoration of state in the event of server failover in which the SMB2 server loses volatile state during a server reboot or failover of a cluster. State information associated with existing opens is lost and must be reestablished. In addition, the resume key is an application-level concept that can only be created and used within the boundary of an application but not shared.